#include <stdio.h>

int main() {
    int n;
    scanf("%d", &n);
    int q = 0, flag = 1;
    double sum = 0.0;
    for (int i = 1; i <= n; i++)
    {
        if (flag % 2 == 0)
        {
            q = q - (2 * i - 1);
            flag++;
            sum += 1.0 / q;
        }
        else {
            q = q + (2 * i - 1);
            flag++;
            sum += 1.0 / q;
        }
    }
    printf("%.3lf", sum);

    return 0;
}